The present invention relates to antenna tracking systems for accurately positioning an antenna toward a radiation source, and more particularly for tracking a beaconless communication transmitter, for example, in a frequency-hopping spread-spectrum communication system.
Spread spectrum communication has been described as a means of data communication in which the signal occupies a larger bandwidth than the minimum necessary to send the information. The spreading of the signal over the band is accomplished by means of a code which is independent of the data, and a synchronized reception with the code at the receiver is used for despreading and subsequent data recovery. Theory of Spread-Spectrum Communications--A Tutorial, R. L. Pickholtz, D. L. Schilling and L. R. Milstein, IEEE Trans. Commun., Vol. COM-30, pp. 855-884, May, 1982.
In frequency hopping spread spectrum communication systems, such as time division multiple access (TDMA) systems, agile synchronization pulses, i.e., pulses whose frequency may vary within a frequency band of interest, are placed in a predetermined manner within that frequency band. The conventional purpose of these synchronization pulses is to allow the receiver to acquire and track the frequency and phase of the communication signal. The synchronization pulses themselves typically include no data messages.
Typically, the approximate location of a mobile spread spectrum transmitter is knowm. This would allow the receive antenna to coarsely track the transmitted signal. It is understood that, typically, the receive antenna comprises an omnidirectional antenna or a fixed directional antenna, so that a tracking system may not be required. Insofar as is known to applicant, the conventional frequency-hopping communications systems without beacons do not employ means for achieving fine or closedloop receive antenna tracking of the frequency-hopping signal. As a result, the communication link performance is not optimized, and may suffer degradation due to signal fading, poor signal tracking and the like.
In contrast with frequency-hopping spread spectrum communication systems, antenna tracking has long been employed in tracking radar applications, such as monopulse radars. The operation of monopulse radars are generally discussed in the Radar Handbook, edited by Merrill I. Skolnik, McGraw-Hill Book Company, 1970, at Section 21.4, pages 21-10 et seq.
The monopulse receiver typically is fed with radar return signals from a directional antenna system, comprising, for example, a multiple horn arrangement coupled to an antenna feed network. An amplitude-comparison type feed network is adapted to provide sum and difference (or error) signal patterns, as are well-known to those skilled in the art. The sum and difference signals are processed to provide error correction signals for correcting the antenna position.
The same tracking technique might be considered for communication signals employing a beacon or single frequency carrier signal, which can be used as a narrowband reference signal. Extension of the technique to frequency-hopping communication systems has not heretofore been achieved, however. For satisfactory operation, the phase relationship between the sum and difference channels would have to be maintained over the entire frequency band of interest. Thus, the frequency response characteristics of the elements comprising the respective sum and difference channels would have to be matched in performance over a much wider frequency band than is required of the tracking of a narrowband beacon, which would be difficult and expensive to achieve.
One tracking scheme that is believed to have been considered for frequency-hopping communication systems is a mechanical scanning system in which tracking information is obtained by rotating or nutating the feed or the main reflector, or by "wobbling" the subreflector in a Cassegrain system. This mechanical scheme tends to add bulk and weight to the receive antenna system. A further limitation is in the low scanning rate which degrades the tracking due to signal scintillation and fading.
It would therefore represent an advance in the art to provide an antenna tracking capability for frequency-hopping spread spectrum communication systems.
It would further be advantageous to provide a relatively simple, yet effective tracking system for wideband communication systems not employing a beacon signal.